Heat transfer device and system

ABSTRACT

Heat transfer device including: a first reservoir (R 1 ) for storing a diphasic fluid (LC), equipped with first heating means (MC 1 ) and connected to a cold source (SF) via a first thermal resistance (RTH 1 ); a second reservoir (R 2 ) for storing said diphasic fluid, equipped with second heating means (MC 2 ) and connected to said cold source or to another cold source via a second thermal resistance (RTH 2 ); and a fluidic pipe (CF) through which said diphasic fluid may pass, connecting said first and second reservoirs, said pipe including at least: an evaporator (EV) that may be thermally connected to a hot source (PC, O) at a temperature higher than that of said cold source; a first condenser (C 1 ) and a second (C 2 ) condenser situated on either side of said evaporator and adapted to be thermally connected to said cold source. 
     Heat transfer system including at least one such device. 
     Method for cooling or precooling an object by means of such a device or system.

The invention relates to a device and a system, in particular a cryogenic device and a cryogenic system, for transfer of heat and to a method of cooling or precooling an object by means of such a device or such a system. The device and the system of the invention notably enable a thermal link of high thermal conductivity to be produced between a cold source and an object to be cooled (hot source).

Controlling the temperature of an object to be cooled and/or giving off heat necessitates the use of a thermal link connecting the object to a “cold source” which, as a function of various technological constraints, may be far removed. At low temperature, i.e. at a temperature below ambient temperature, controlling the temperature of the object necessitates two essential thermal functions, namely:

to enable cooling of the object, initially at ambient temperature, to its operating temperature, i.e. the temperature required for the correct functioning of the object; this is the “precooling” function;

to maintain its temperature by a transfer of heat to the cold source at the operating temperature; this is the “thermal stabilization” function.

In some applications, in particular in the space field, it is of benefit also to have an additional “thermal switch” function that consists in interrupting the transfer of heat by an external action.

Transfer of heat over a distance that may reach several meters may be effected thanks to various devices.

The simplest of these and the most widely used is the metal braid operating by simple conduction of heat in a solid of high thermal conductivity. The major drawback of such a device is that it has a high mass, especially if a low thermal resistance is required. Moreover, a thermal gradient between the cold source and the object to be cooled cannot be avoided. Furthermore, the “thermal switch” function cannot be assured.

Convection devices, exploiting the circulation of a fluid driven by a pump, therefore appear much more attractive in numerous applications, notably cryogenic applications. In most cases, for reasons of reliability, mass and overall size, one wishes to avoid the use of mechanical pumps in which mechanical elements move relative to each other. Pumping with no moving parts is thus preferred. Moreover, diphasic systems are generally employed in which the transfer of heat is effected by evaporating a liquid on the object side and condensing the vapor on the cold source side. In principle this type of system operates with no temperature difference and enables high heat transfer by way of liquid/vapor phase change because of the high latent heat of the phase change, which accounts for its benefit. Four principal types of thermal link operate on this principle: the thermosiphon, the “simple” heat pipe, the fluid loop and the pulsating heat pipe. These are passive devices, i.e. devices in which the fluid circulates without external action.

In a thermosiphon the diphasic fluid circulates because of the effects of gravitational forces induced by the density difference between the liquid and the vapor. The system may be a simple passage in which the descending liquid and the rising vapor circulate in counterflow, or a loop with a liquid passage and a vapor passage. In all cases, the object must be situated lower down than the cold source, which constitutes a constraint that is sometimes unacceptable. Moreover, given its operating principle, the thermosiphon does not work in a microgravity situation and is therefore not suitable for space applications.

The “simple” heat pipe and the fluid loop make it possible to avoid these constraints by exploiting “capillary pumping” generated by the curvature of a liquid meniscus evaporating within a capillary structure.

The “simple” heat pipe is a tube containing a capillary structure, commonly called a “wick”, in thermal contact with the internal walls of the tube. The liquid is located only in the wick and flows from the condenser to the evaporator. The vapor produced returns in contraflow to the condenser, in a passage provided at the center of the tube.

Just as in the case of the thermosiphon, the “thermal switch” function cannot be provided in a simple way; on the other hand, reliable operation in a microgravity situation is possible. Used more with a rectilinear geometry to homogenize the temperature of an object, its integration into a complex thermal control system may be problematic, because of its necessitating non-rectilinear geometries. Moreover, the heat pipe is not suitable for transporting heat over long distances (several meters) because of the high head losses generated by the flow of the liquid in the porous wick and by viscose interactions between the liquid and the vapor (driving losses).

The fluid loop, commonly called a Loop Heat Pipe (LHP) or Capillary Pumped Loop (CPL), makes it possible to overcome the aforementioned drawbacks of the “simple” heat pipe. It operates on the basis of the same principle as the latter, but the capillary structure is located uniquely at the level of the evaporator in order to generate the capillary pumping necessary for the circulation of the fluid; moreover, the evaporator and the condenser are connected by two separate pipes for the liquid and the vapor. In this way, head losses are lower, heat transfer may be effected over long distances, and recourse to non-rectilinear geometries proves less problematic. The “thermal switch” function may be effected by simple localized heating on the liquid line that causes the liquid to boil and brings about unpriming of the loop.

A more detailed description of the operation of a fluid loop may be found in the paper by Jentung Ku “Operating Characteristics of Loop Heat Pipes”, 29th International Conference on Environmental Systems, 12-15 Jul. 1999, Denver, USA.

The use of fluid loops at cryogenic temperatures gives rise to the problem of precooling of the object (“hot source”) in thermal contact with the evaporator. For capillary pumping to be primed, it is necessary for the wick to be engorged with the cryogenic fluid in the liquid state. However, in a fluid loop, the wick is located only in the evaporator, i.e. in the “hot” part of the device. Precooling is therefore necessary to enable wetting of the wick. A number of solutions have been proposed for producing cryogenic LHP or CPL.

A first cryogenic CPL concept known as the Cryogenic Capillary Pumped Loop (CCPL) is described in the paper by D. Bugby and B. Marland “Flight results from the Cryogenic Capillary Pumped Loop (CCPL) Flight Experiment on STS-95” SAE paper No. 981814, 28th International Conference on Environmental Systems, Jul. 13-16, 1998, [Danvers], USA. In this device, precooling is effected by the expulsion toward the evaporator a cryogenic liquid contained in a cold reservoir via a precooled reservoir line in a heat exchanger (“condenser spool”). This expulsion is effected by electrical heating on the reservoir. The arrival of liquid at the evaporator causes cooling thereof and finishes by filling it with liquid. The system is then ready to be primed.

Another concept consists in inserting into the principal loop a secondary capillary pump hydraulically connected to a secondary condenser thermally connected to the cold source. Application of electrical heating to the secondary pump generates the circulation of the fluid and consequently the feeding with liquid of the principal evaporator, which leads to its precooling. This concept is described in the following publications:

-   D. Khruslatev, “Cryogenic loop heat pipes as flexible thermal links     for cryocoolers”, Proc. 12th Cryocoolers Conference, pp. 709-716     (2003); -   Q. Mo and J. T. Liang, “A novel design and experimental study of a     cryogenic loop heat pipe with high heat transfer capability” IJHMT     49, pp 770-776 (2006); and -   Q. Mo, J. T. Liang and C. Jinghui, “Investigation of the effects of     three key parameters on the heat transfer capability of a CLHP”,     Cryogenics 47 pp. 262-266 (2007).

A further solution consists in using a secondary circuit including a secondary fluid line, a condenser, a diphasic reservoir and a secondary capillary pump. By simple application of electrical power to the secondary pump, this circuit enables precooling of the loop, in particular filling of the principal evaporator with liquid. This solution is described in the paper by J. Yun, E. Kroliczek and L. Crawford “Development of a Cryogenic Loop Heat Pipe (CLHP) for Passive Optical Bench Cooling Applications”, 32nd ICES 2002, SAE paper n° 2002-01-2507, San Antonio, Tex., 2002, and in U.S. Pat. No. 7,004,240.

A similar concept is that of the cryogenic advanced LHP: see T. T. Hoang, D. Khruslatev and J. Ku, “Cryogenic advanced loop heat pipe in temperature range of 20-30K” Proc. 12th International heat pipe conference, (2002), pp. 201-205; US 2003/0159808; WO 03/054469, 3 Jul., 2003.

A further possibility consists in using gravity to prime the loop. The paper by H. Pereira, F. Haug, P. Silva, J. Wu, and T. Koettig, “Cryogenic loop heat pipe for the cooling of small particle detectors at CERN”, Cryogenic Engineering Conf., 28 Jun.-2 Jul. 2009, Tucson, USA, describes an LHP in which the liquid line is placed above the evaporator; in this way, “gravity pumping” enables priming and assists capillary pumping in normal operation. This principle is not suited to space applications, of course.

Another diphasic passive heat transfer device is the Pulsating Heat Pipe (PHP). This device is constituted by a simple tube, with a diameter less than the capillary length, forming a plurality of loops or undulations and filled with a diphasic fluid constituted by liquid, forming “liquid plugs”, and vapor, forming bubbles. One end of each loop or undulation is brought into thermal contact with a hot source and the opposite and with a cold source. Under these conditions instability is created causing oscillatory movement of the liquid plugs or bubbles. The result of this is extremely efficient thermal transfer. The capillary may be closed at both ends (“open” PHP) or be looped on itself (“closed” PHP, more effective).

The PHP principle is described in the paper by M. B. Shafi, A. Faghri and Y. Zhang “Analysis of heat transfer in unlooped and looped pulsating heat pipes”, Int. Journ. of Numerical Methods for Heat & Fluid Flow, Vol. 12, No. 5, (2002), pp. 585-609.

A PHP suited to cryogenic applications is described in the paper by R. Chandratilleke et al., “Development of cryogenic loop heat pipe”, Cryogenics 38 (1998) pp. 263-269. Although precooling is necessary, it is not referred to in this publication.

Diphasic heat transfer devices such as those described above may generally be qualified as “passive” when they accomplish their thermal stabilization function; however, when they are used at cryogenic temperatures, they necessitate precooling means—often active means—which considerably complicate their structure and operation and/or that impose constraints that are sometimes unacceptable (presence of a gravitational field, relative arrangement of certain components).

The invention aims to overcome the aforementioned drawbacks of the prior art by proposing a cryogenic heat transfer device of particularly simple structure enabling precooling of an object, in an independent manner and including against gravity, over large distances, but also evacuation of the heat given off by the object once cooled. This device may be qualified as “active” because it employs actuators (heat sources) to assure the circulation of a diphasic fluid; however, no mechanical moving parts are necessary (except valves in some embodiments). It is able on its own to provide the precooling and thermal stabilization functions or be used only for the step of precooling an object, in which case thermal stabilization may be assured by a conventional passive device.

One object of the invention is therefore a heat transfer device, notably a cryogenic heat transfer device, including:

a first reservoir for storing a diphasic fluid, equipped with first heating means and connected to a cold source via a first thermal resistance;

a second reservoir for storing said diphasic fluid, equipped with second heating means and connected to said cold source or to another cold source via a second thermal resistance; and

a fluidic pipe able to be traversed by said diphasic fluid, connecting said first and second reservoirs, said pipe including at least:

an evaporator able to be thermally connected to a hot source at a temperature higher than that of said cold source;

a first condenser and a second condenser situated on either side of said evaporator and able to be thermally connected to said cold source; said first and second heating means and said fluidic pipe being arranged in such a manner that activation of the first heating means causes expulsion of said diphasic fluid from said first reservoir toward said second reservoir via said fluidic pipe and activation of the second heating means causes expulsion of said diphasic fluid from said second reservoir toward said first reservoir via said fluidic pipe.

In different embodiments of the invention:

The device may contain a diphasic fluid in an amount at least sufficient, in the liquid state, to fill said fluidic pipe and part of the volume of one of said first and second reservoirs, but insufficient, in the liquid state, to fill both reservoirs and said fluidic pipe.

Said first and second reservoirs may have a capacity greater than that of the fluidic pipe. These reservoirs may preferably have the same capacity.

Said diphasic fluid may be a cryogenic fluid having a critical temperature less than or equal to 200K, or even 120K. It may for example be helium, hydrogen, neon, nitrogen or oxygen at respective temperatures of 4.2K, 20K, 27K, 77K and 90K and the critical temperatures of which are respectively 5.2K, 33K, 44K, 126K and 154K.

The device may further include at least one cold source including cooling means adapted to bring it to a temperature enabling the existence of a liquid phase of said fluid inside said reservoirs.

Said fluid pipe may be connected to said first and second reservoirs via respective bleeds produced at the lower ends thereof. This embodiment is suitable for terrestrial applications, in the presence of a gravitational field.

Alternatively, each of said first and second reservoirs may contain a thermally conductive porous material wettable by the liquid phase of said fluid; by “wettable” material is meant a material with which said liquid phase forms a contact angle less than 90°. This embodiment is suitable for space applications in a microgravity environment.

Said fluid pipe may be connected to a pressure reduction reservoir. This is a feature that is advantageously present in most heat transfer fluidic devices operating at cryogenic temperatures.

The device of the invention may further include a control device adapted to activate alternately the first heating means and the second heating means in such a manner as to cause a transfer of said diphasic fluid from said first reservoir to said second reservoir and vice versa.

The invention also provides a heat transfer system including two devices as described above thermally connected between said hot source and said cold source, or respective cold sources, wherein said control means are configured to activate the respective heating means periodically and in phase quadrature.

The invention further provides a heat transfer system including a device as described above and a heat transfer passive diphasic device, such as a fluidic loop heat pipe or a pulsating heat pipe, thermally connected between said hot source and said cold source, or respective cold sources. In one particular embodiment said heat transfer passive diphasic device may be connected to said first and second reservoirs via a system of valves enabling it to be filled with diphasic fluid.

The invention further provides a heat transfer system including a device as described above wherein a pulsating heat pipe thermally connected between said hot source and cold source is integrated into said fluidic pipe.

The invention further provides a method of cooling or precooling an object, notably to a cryogenic temperature, by means of a device as described above, including the following steps:

a. Thermally connecting said object to the evaporator of said device so that it functions as a hot source; b. Thermally connecting said first and second reservoirs and said first and second condensers to said cold source in such a manner as to cause at least partial filling of at least said first reservoir with a liquid phase of said diphasic fluid; c. Activating said first heating means so that said liquid phase of said diphasic fluid flows toward said second reservoir via said evaporator, where it is evaporated at least partially, cooling said object, and said second condenser, where the vapor formed in this way returns to the diphasic state; d. Deactivating said first heating means when the first reservoir is substantially empty of said liquid phase; e. Activating said second heating means so that said liquid phase of said diphasic fluid flows toward said first reservoir via said evaporator, where it is evaporated at least in part, cooling said object, and said first condenser, where the vapor formed in this way returns to the diphasic state; and f. Deactivating said second heating means when the second reservoir is substantially empty of said liquid phase; the steps c. to f. being repeated cyclically.

The invention further provides a method of cooling an object including a precooling step as described above followed by a step of thermal stabilization by means of a heat transfer passive diphasic device.

Other features, details and advantages of the invention will emerge from a reading of the description given with reference to the appended drawings provided by way of example and in which:

FIGS. 1A, 1B and 1C are functional diagrams of three heat transfer devices of three variants of one embodiment of the invention;

FIGS. 2A and 2B are two views in section of a cryogenic fluid reservoir suitable for use under microgravity conditions;

FIG. 3 is the functional diagram of a heat transfer cryogenic system of a first embodiment of the invention employing two devices of the type represented in FIG. 1A connected in parallel with a fluid loop between a hot source and a cold source;

FIG. 4 is the functional diagram of a heat transfer cryogenic system of a second embodiment of the invention employing a device of the type represented in FIG. 1A connected in parallel between a hot source and a cold source;

FIG. 5 is the functional diagram of a heat transfer cryogenic system of a third embodiment of the invention employing a device of the type represented in FIG. 1A and a closed pulsating heat pipe connected in parallel between a hot source and a cold source;

FIG. 6 is the functional diagram of a heat transfer cryogenic system of a fourth embodiment of the invention employing a device of the type represented in FIG. 1A with which an open pulsating heat pipe is integrated;

FIGS. 7A-7C are diagrams showing the structure and the operation of a heat transfer cryogenic system of a fifth embodiment of the invention employing a device of the type represented in FIG. 1A and an open pulsating heat pipe connected in parallel between a hot source and a cold source; and

FIGS. 8A and 8B show experimental results illustrating the operation of a device shown in FIG. 1A.

As FIG. 1A shows, a heat transfer device of the invention essentially comprises two reservoirs R1 and R2, preferably having the same capacity, interconnected by means of a fluidic pipe CF. This assembly contains a fluid in the diphasic state, liquid and vapor. It is constituted, in the direction R1 to R2, of a first condenser C1 (dark grey), a first section of the fluidic pipe, an evaporator EV, a second section of the fluidic pipe (light grey), and a condenser C2.

The diphasic fluid LC has a critical temperature lower than the operating temperature of the object O to be cooled, and is partly in the liquid state at the temperature of the cold source. As a function of the application concerned, it may for example be water, ammonia or a cryogenic fluid such as liquid nitrogen, oxygen, hydrogen, neon or helium. The device of the invention is particularly suitable for cryogenic applications (temperatures of the object less than or equal to 200K or even 120K), or more generally applications in which the object to be cooled must be brought to an operating temperature less than ambient temperature (by convention 20° C.). The quantity of diphasic fluid contained in the device must be sufficient, in the liquid state, to fill the fluidic pipe and at least part (typically 50% or 75%) of the internal volume of one of the reservoirs. At the same time, the device must not be entirely filled with liquid, because in this case no circulation of the diphasic fluid could occur.

The first condenser C1, which is in the immediate vicinity of the first reservoir R1, is in thermal contact with a cold source SF, having means (for example a bath of cryogenic fluid or a cryo-refrigerator) able to bring its temperature T_(F) to a value less than or equal to the saturation temperature of the diphasic fluid. Thus this condenser C1 is filled with diphasic fluid in the liquid state.

The evaporator EV, located in the central part of the fluidic pipe CF, is in thermal contact with a “hot plane” PC, which is an element that is a good conductor of heat that is thermally connected to an object O to be cooled. The hot plane PC serves as a “hot source”; its temperature T_(C) is greater than or equal to the saturation temperature of the diphasic fluid. Thus the evaporator EV contains fluid in the liquid state, then diphasic, then—possibly—entirely in the vapor state.

The first reservoir R1 is connected to the cold source via a first thermal resistance RTH1. Similarly, the second reservoir R2 is connected to the cold source via a second thermal resistance RTH2. The values of these resistances constitute parameters that are important for the rating of the device of the invention, as will be discussed hereinafter. Moreover, the two reservoirs are equipped with respective heating means MC1, MC2, for example electrical resistances. A control device DC (computer, microprocessor card, etc.) emits signals sMC1, sMC2 for controlling the two heating means MC1 and MC2.

A pressure reducing “hot” reservoir RRP is connected to the fluidic pipe CF. This is a conventional feature of cryogenic systems, intended to prevent an excessive rise in pressure when the system is at ambient temperature. In other embodiments, the reservoir RRP may be absent: in this case, the device is pressurized at ambient temperature, in the supercritical domain; cooling it then takes a long time, because the fluid must be cooled first by conduction in the gas, before being condensed in the cold parts of the system.

To describe the operation of the FIG. 1A device, there is considered the initial situation in which the device, apart from the evaporator EV, is “cold”, at the temperature T_(F). The two reservoirs R1 and R2 are partly filled with liquid with vapor above. The two condensers C1 and C2 are totally full of liquid, while the rest of the pipe, including the evaporator EV, is filled with vapor. At the beginning, the heating means MC1, MC2 are off and the system is “thermalized”: the reservoirs R1, R2 and condensers C1, C2 are at the temperature T_(F) of the cold source while the evaporator EV is at the temperature T_(C) of the hot plane.

At the time t=t₀ the first heating means MC1 are activated to inject heat into the first reservoir R1. This causes evaporation of a small part of the liquid that is contained therein, and thus an increase in pressure that causes the expulsion of another great part of the liquid in the pipe CF to the second reservoir. The liquid is cooled in the condenser C1. Because of the effects of the increased pressure induced by heating, the liquid is then directed toward the evaporator EV where it is heated, and then evaporates partly or totally. In the latter case, the vapor is superheated, i.e. its temperature is higher than the saturated vapor temperature T_(SAT) at the pressure that reigns in the fluidic pipe, at the outlet from EV. This being so, the fluid extracts heat from the hot plane PC and the object O. The vapor (or the diphasic (liquid/vapor) fluid) that leaves the evaporator continues to flow toward the second reservoir R2. Before reaching it, however, it passes through the second condenser C2, where it gives up heat to the cold source on condensing. At the outlet from C2, the fluid is diphasic. The vapor phase component of this fluid, entering the reservoir R2, is condensed thanks to the cold power passing through the thermal resistance RTH2. The incoming liquid thus fills the reservoir R2.

Over time, the first reservoir R1 is emptied of the liquid component of the diphasic fluid. Given that the outlet bleed from the reservoir R1 is situated in the lower part, when the liquid level falls below this bleed, the reservoir is virtually empty. Pure vapor leaves R1, which is depressurized; consequently, its temperature falls. This temperature drop is detected and constitutes the signal that triggers the Deactivating of MC1 and the turning on of MC2. From this moment, the reservoir R2, which has been partially filled with liquid, becomes the “source” reservoir, while R1 becomes the “recovery” reservoir. The flow of fluid in CF is reversed. R2 is emptied because of the effect of MC2. The cycle terminates when R2 is virtually empty. A new cycle may then begin to be repeated as necessary.

Alternatively, the signal triggering the Deactivating of MC1 and the turning on of MC2 could be the increase in the temperature of the reservoir R1 that occurs after the latter is completely emptied of liquid.

The principle criteria for rating a device of the type shown in FIG. 1A are as follows:

A portion Q_(RS) _(—) _(F) of the heating power Q_(RS) injected into the source reservoir (R1 or R2, as a function of the operating phase) is lost via the thermal resistance RTH1, RTH2. The difference Q_(RS)−Q_(RS) _(—) _(F) serves to generates the flow rate {dot over (m)} of fluid in the fluidic pipe, and to generate by evaporation the vapor replacing the liquid that leaves the source reservoir. Maximizing the thermal resistances RTH1 and RTH2 enables limitation of the power lost and thus improvement of the energy efficiency of the device, but increases the cooling time, i.e. the time necessary to reach the initial conditions described above. The power {dot over (Q)}_(R1) _(—) _(F), {dot over (Q)}_(R2) _(—) _(F) lost via the thermal resistors RTH1 and RTH2 has the value:

${\overset{\cdot}{Q}}_{R\; {1/2}{\_ F}} = {\frac{T_{SAT} - T_{F}}{R_{{TH}\; {1/2}}}.}$

The power exchanged in the evaporator EV has the value:

Q _(EV) ={dot over (m)}[c _(PL)(T _(SAT) −T _(F))+h _(LV) +c _(PV)(T _(VC) −T _(SAT))]

where {dot over (m)} is the flow rate in the fluidic circuit, c_(PL) and c_(PV), respectively, the specific heat of the liquid phase and the vapor phase at constant pressure, h_(LV) the latent heat of evaporation, T_(VC) the temperature of the fluid in the pipe on the recovery reservoir side.

The flow rate {dot over (m)} in the fluidic circuit has the value:

$\overset{\cdot}{m} = {\frac{{\overset{\cdot}{Q}}_{RS} - {\overset{\cdot}{Q}}_{R\; {1/2}{\_ F}}}{h_{LV}}\left( {\frac{\rho_{l}}{\rho_{v}} - 1} \right)}$

where ρ₁ and ρ_(v) are, respectively, the densities of the liquid phase and the vapor phase when saturated at the temperature of the reservoirs, which are assumed equal. This flow rate is essentially imposed by the exchanged power value Q_(EV) required by the application concerned.

The flow rate {dot over (m)} in CF being given, the fluid leaving C_(1/2) enables cooling of the object. The fluid leaves the evaporator EV at a temperature T_(VC) less than but close to T_(O). The fluid, if it is in the vapor state, is cooled on a very small area and is condensed on virtually all of the cold area that is at T_(F). Almost all of the power is exchanged via this area. To a first approximation Q_(EV)≈H_(COND)S_(C2/1)(T_(SAT)−T_(F)) where H_(CONL) and S_(C2/1) are respectively the condensation heat transfer coefficient and the exchange area S_(C2/1) of the recovery condenser C_(2/1). This area is therefore fundamental to the rating process. It fixes the saturation temperature, i.e. the temperature of the reservoirs and thus their pressure.

FIGS. 1B and 1C relate to variants of the FIG. 1A device. In the case of FIG. 1A, the two condensers are integrated into the same part, disposed between the reservoirs and the cold source. In the case of FIG. 1B, the condensers are independent of each other, but again disposed between the reservoirs and the cold source. In the case of FIG. 1C, the two condensers are independent of each other and the reservoirs.

FIGS. 8A and 8B show experimental results illustrating the operation of a device of the FIG. 1C type, using helium as diphasic fluid and a cold source constituted by a cryogenic bath of helium (T_(F)=4.3K approx). FIG. 8A shows the evolution of the temperature T_(C) of the hot plane, which passes from 7 0K to 4.3K in less than an hour, for a thermal mass of 400 J. FIG. 8B shows the fluctuations of the temperatures T_(R1), T_(R2) of the reservoirs and the much smaller fluctuations of the temperature T_(F) of the cold source.

The FIG. 1A example relates to the case of a device operating in a gravitational field. Under these conditions, the heating means MC1 and MC2 are preferably located in the upper part of each reservoir, while the bleeds connecting the pipe CF to the reservoirs is located in the lower part of the latter. This arrangement makes it possible to assure that the increase in pressure in the reservoir causes an injection of liquid, and not of vapor, into the pipe CF.

In the absence of gravity (space applications) there arises the problem of locating the liquid/vapor interface, which is necessary to assure that only liquid is injected into the fluidic pipe CF. The solution shown in FIGS. 2A and 2B consists in using a porous material MP that can be wetted by the cryogenic liquid to be engorged thereby, completely (or almost completely) filling each reservoir R. In the example of FIGS. 2A/2B, the heating means MC are situated at the center of the reservoir, in contact with the porous material. When these heating means are activated, a temperature gradient is created in the porous material, with temperatures higher than the saturated vapor temperature (i.e. the boiling or liquefaction point of the fluid) at the center and lower at the periphery. This imposes that the vapor be at the center, close to the heating means, and the liquid in the peripheral part of the reservoir. The increase in the pressure in the vapor, caused by evaporation in a closed volume, constrains the liquid to escape via peripheral grooves RP provided for this purpose. The use of a conductive porous material (for example a metal) is preferable for the flow of heat to go directly to the liquid/vapor interface, instead of generating a high temperature gradient that would be of no utility.

Of course, other geometries are possible; for example, the heating means may be disposed at one end of the reservoir and the bleed for the pipe CF at the opposite end.

The FIG. 1A device may be used on its own as a thermal link enabling precooling of the object O from an arbitrary high temperature toward the temperature of the cold source, and maintaining it at a low temperature (thermal stabilization). Thanks to the active character of the device, the thermal switch function is implemented very simply: it suffices not to activate the reservoir heating means.

The device may equally constituent a component of a more complex heat transfer cryogenic system.

A first example of such a system is shown in FIG. 3. This system is constituted by two devices as in FIG. 1A, identified by the references Da, Db. The various components of these devices are identified by the letters “a” and “b”; for example, “R1 a” is the first reservoir of the device “a”, and so on. The control devices Dca, DCb transmit signals sMC1 a/sMC2 a, sMC1 b/sMC2 b for controlling the heating means MC1 a/MC2 a, MC1 b/MC2 b which are in “phase quadrature”, i.e. offset temporally by one quarter (or three quarters, which amounts to the same thing) of the duration of a complete cycle. The two devices are ideally identical and have equal cycle times.

If the power Q_(EV) given off by the objet O is constant in time, heating must be managed so that the sum of the flow rates {dot over (m)}_(a)+{dot over (m)}_(b) is also constant. The temperature of the object will then be stable. On the other hand, if the power Q_(EV) is not stable in time, it is necessary to vary the sum of the flow {dot over (m)}_(a)+{dot over (m)}_(b) rates by means of appropriate regulation.

The two control devices Dca, DCb may be produced in the form of a single device.

In systems conforming to other embodiments of the invention, the FIG. 1A device is used for precooling of the object O and the hot plane PC, the thermal stabilization function being assured by a passive device of conventional type connected in parallel between the cold source SF and said hot plane.

FIG. 4 shows one such system, in which the thermal stabilization function is assured by a fluid loop LHP including an evaporator EVc, in thermal contact with the hot plane PC and containing the capillary width M, a compensation chamber CC disposed upstream of said evaporator, a fluidic pipe CFc connected to a pressure reduction hot chamber PRPc. In the FIG. 4 system, the device of the invention is used to precool the hot plane to a temperature enabling the presence of liquid in the compensation chamber and in the evaporator of the fluid loop LHP. Once primed, the latter takes over.

In the FIG. 5 embodiment, the function of thermal stabilization after precooling is assured by a closed type pulsating heat pipe PHPF.

In the embodiments of FIGS. 3 to 5, each device is equipped with its own pressure reduction reservoir RRP, RRPa, RRPb, RRPc, RRPd. As a function of the operating regime of the system, these reservoirs may be at different temperatures. The use of a common “hot” reservoir would necessitate the use of a complex system of valves.

In the system shown in FIG. 6, an open pulsating heat pipe PHPO is “integrated” into the fluid pipe CF of a device of the invention. The evaporator EV is thus divided into a plurality of hot regions of the pulsating heat pipe, alternating with cold regions thereof. The system operates in the manner explained above with reference to FIG. 1A during the precooling phase; the heating means MC1, MC2 are then deactivated and the passive PHP takes over for the stabilization phase. This concept is beneficial because it is more compact than that of FIG. 5 (in particular, only one pressure reduction reservoir is needed), and because the PHP may be filled directly from the reservoirs R1 and R2. It is also subject to certain constraints, however:

firstly, the fluidic pipe CF—or at least its central part forming the pulsating heat pipe—must be of capillary type (diameter less than a few times the capillary length of the liquid) and very long, which increases the head losses. It follows that the temperature T_(RS) of the source reservoir must be higher than in the case of a “simple” device such as that from FIG. 1A, with the resulting increase in the leakage thermal flow;

secondly, the pulsating heat pipe must be of the open type, less efficient than the closed PHP of FIG. 5;

thirdly, there is no assurance that the volume fraction of the liquid phase will be close to the optimum value of 50% for correct operation of the PHP;

fourthly, the large number of round trips of the fluidic pipe of the PHPO between the cold source and the hot source imposes the provision of a large reservoir volume.

These drawbacks may be avoided, at least in part, thanks to the system of FIGS. 7A-7C, in which the device of the invention is an integral part of a closed pulsating heat pipe. The other side of the coin is the use of two three-port valves V3V1, V3V2, and thus of mechanical elements having moving parts.

The system from FIGS. 7A-7C comprises a device D of the type shown in FIG. 1A and a pulsating heat pipe PHPF′ mounted in parallel between the cold source and the hot plane. The two ends of the pulsating heat pipe are connected to the first and second condensers of the device via the three-port valves V3V1, V3V2; in this way, the heat pipe is looped on itself via the fluidic pipe CF.

Initially (FIG. 7A) the valves are in a first position isolating the pulsating heat pipe, which is filled with vapor. The device D operates in the manner described above to precool the hot plane PC and the object O.

Once precooling has finished, the valves go to a second position in which they connect the pulsating heat pipe to the two reservoirs R1, R2 of the device D (FIG. 7B). Thus activation of the heating means of the source reservoir (R1 in this case) causes expulsion of liquid therefrom and filling of the pulsating heat pipe.

Finally (FIG. 7C), the valves go to a third position in which the fluidic pipe CF of the device D is connected to the pulsating heat pipe to form a supplementary loop or undulation thereof. The heating means are inactive and the system operates in a passive manner, like a standard pulsating heat pipe.

In the case of FIGS. 7A-7C, the fluidic pipe CF is capillary, as shown by the alternating liquid plugs and bubbles visible in FIG. 7C; however, its length is much less than that of the FIG. 6 pipe (which of itself forms a pulsating heat pipe), and consequently the head losses are lower.

In another embodiment, not shown, the device of the invention could be used for precooling and filling a fluidic loop of CPL or LHP type.

Until now the situation has always been considered in which there is only one cold source and one hot plane/object to be cooled. This is not an essential limitation, it is of course entirely possible to use a separate cold source for each device or reservoir, for example, although this complicates the control of the heating means.

It is also possible to envisage more complex systems, including one or more devices of the invention cooperating with each other (as in the case of FIG. 3) and/or with one or more heat transfer devices of different types (as in the case of FIGS. 4 and 5). Devices more complex than that of FIG. 1 are also conceivable, including more than two reservoirs and a plurality of fluidic pipes, condensers and evaporators. 

1. A heat transfer device including: a first reservoir for storing a diphasic fluid, equipped with first heating means and connected to a cold source via a first thermal resistance; a second reservoir for storing said diphasic fluid, equipped with second heating means and connected to said cold source or to another cold source via a second thermal resistance; and a fluidic pipe able to be traversed by said diphasic fluid, connecting said first and second reservoirs, said pipe including at least: an evaporator able to be thermally connected to a hot source at a temperature higher than that of said cold source; a first condenser and a second condenser situated on either side of said evaporator and able to be thermally connected to said cold source; said first and second heating means and said fluidic pipe being arranged in such a manner that activation of the first heating means causes expulsion of said diphasic fluid from said first reservoir toward said second reservoir via said fluidic pipe and activation of the second heating means causes expulsion of said diphasic fluid from said second reservoir toward said first reservoir via said fluidic pipe.
 2. The heat transfer device claimed in claim 1, containing a diphasic fluid in an amount at least sufficient, in the liquid state, to fill said fluidic pipe and part of the volume of one of said first and second reservoirs, but insufficient, in the liquid state, to fill both reservoirs and said fluidic pipe.
 3. The heat transfer device claimed in claim 1, wherein said and second reservoirs have a capacity greater than that of the fluidic pipe.
 4. The heat transfer device claimed in claim 1, wherein said first and second reservoirs have the same capacity.
 5. The heat transfer device claimed in claim 1, wherein said diphasic fluid is a cryogenic fluid having a critical temperature less than or equal to 200K.
 6. The heat transfer device claimed in claim 1 further including at least one cold source including cooling means adapted to bring it to a temperature enabling the existence of a liquid phase of said fluid inside said reservoirs.
 7. The heat transfer device claimed in claim 1 wherein said fluid pipe is connected to said first and second reservoirs via respective bleeds produced at the lower ends thereof.
 8. The heat transfer device claimed in claim 1 wherein each of said first and second reservoirs contains a thermally conductive porous material, wettable by the liquid phase of said diphasic fluid, in thermal contact with said heating means.
 9. The heat transfer device claimed in claim 1 wherein said fluidic pipe is connected to a pressure reduction reservoir.
 10. The heat transfer device claimed in claim 1 further including a control device adapted to activate alternately the first heating means and the second heating means in such a manner as to cause a transfer of said diphasic fluid from said first reservoir to said second reservoir and vice versa.
 11. The heat transfer system claimed in claim 10 including two devices (Da, Db) thermally connected between said hot source and said cold source, or respective cold sources, wherein said control devices are configured to activate the respective heating means periodically and in phase quadrature.
 12. The heat transfer system claimed in claim 1 including a device and a heat transfer passive diphasic device, such as a fluidic loop heat pipe or a pulsating heat pipe, thermally connected between said hot source and said cold source, or respective cold sources.
 13. The heat transfer system claimed in claim 12 wherein said heat transfer passive diphasic device is connected to said first and second reservoirs via a system of valves enabling it to be filled with diphasic fluid.
 14. The heat transfer system claimed in claim 1, including a device wherein a pulsating heat pipe thermally connected between said hot source and cold source is integrated into said fluidic pipe.
 15. A method of cooling or precooling an object by means of the device claimed in claim 1, including the following steps: a. Thermally connecting said object to the evaporator of said device so that it functions as a hot source; b. Thermally connecting said first and second reservoirs and said first and second condensers to said cold source or to respective cold sources in such a manner as to cause at least partial filling of at least said first reservoir with a liquid phase of said diphasic fluid; c. Activating said first heating means so that said liquid phase of said diphasic fluid flows toward said second reservoir via said evaporator, in which it evaporates at least partially, cooling said object, and said second condenser, where the vapor formed in this way returns to the diphasic state; d. Deactivating said first heating means when the first reservoir is substantially empty of said liquid phase; e. Activating said second heating means so that said liquid phase of said diphasic fluid flows toward said first reservoir via said evaporator, where it is evaporated at least in part, cooling said object, and said first condenser, where the vapor formed in this way returns to the diphasic state; and f. Deactivating said second heating means when the second reservoir is substantially empty of said liquid phase; the steps c. to f. being repeated cyclically.
 16. The method claimed in claim 15 of cooling an object, including a precooling step followed by a step of thermal stabilization by means of a heat transfer passive diphasic device. 